This invention concerns thermoset powder coating compositions, especially for automotive vehicles.
Powder coating compositions have become increasingly important because they give off very little or no volatile material to the environment when cured. Typically, any such emissions are limited to by-products of the curing reaction, such as blocking agents or volatile condensation products. Powder coatings have found use as both decorative coatings and protective coatings.
Topcoats for automotive and other industrial applications may be a one-layer coating, in which the color is generally uniform through the coating layer, or a clearcoat-basecoat composite coating, having a colored basecoat layer underlying a transparent clearcoat layer. Clearcoat-basecoat composite coatings are widely used in the coatings art and are notable for desirable gloss, depth of color, distinctness of image and/or special metallic effects. Composite systems are particularly utilized by the automotive industry to achieve a mirror-like, glossy finish with a high depth of image. All of the coating layers, including the underlying primer layer or layers, should be as smooth as possible to attain the best depth of image.
It is also important for topcoats, including the clearcoat-basecoat composite coatings, to provide the desired color. Automotive bodies are generally first coated with a layer of a cathodic electrocoat primer and/or other primer layer. In the case of cathodic electrocoat primers, the salting amine that remains in the cured electrocoat primer layer may be volatilized during thermal cure of later applied coating layers causing undesirable yellowing in those layers.
Acrylic polymers have been widely used in solventborne and aqueous topcoat coating compositions. Earlier attempts to formulate an acrylic powder coating composition for topcoats, however, have met with difficulties. One problem has been the tendency of the acrylic powder coating to contaminate other coating compositions being used in the manufacturing plant, causing cratering and other appearance problems.
Rehfuss and Ohrbom, in U.S. Pat. No. 5,371,167, describe solventborne coatings, particularly for automotive clearcoats, containing carboxyl-functional crosslinkers having a cyanuric ring and epoxide-functional acrylic polymers. The coating composition of Example 8, in particular, contains an epoxide-functional acrylic polymer and the acid-functional reaction product of 1,3,5-tris(2-hydroxyethyl)cyanuric acid and methylhexahydrophthalic anhydride. The patentees teach that it is necessary to include solvent to obtain flow and leveling in the coating. Solventborne coatings, however, produce regulated organic emissions which require costly abatement procedures and equipment.
Inoue et al., U.S. Pat. No. 6,255,392 describe a topcoat composition containing a vinyl copolymer with hydrolyzable silyl groups and hydroxyl groups, a compound with hydroxyl groups, and solvent. The coating composition may also contain a carboxyl-functional compound, apparently to improve recoating adhesion. The carboxyl-functional compound may be the half-ester of an acid anhydride reacted with a polyol. The carboxyl groups are not reacted in curing the topcoat. The Inoue patent also does not discuss powder coatings or the powder coating problems we have mentioned above.
Ramesh, U.S. Pat. Nos. 5,925,285 and 6,130,297, describes low gloss coatings including 1,3,5-tris-(2-carboxyethyl)isocyanurate, a dicarboxylic acid crosslinking agent, and a polyepoxide resin. Automotive coatings, however, must be glossy. Automotive topcoat coatings must have high gloss for aesthetic reasons. Automotive primers are also formulated to be fairly glossy because, among other reasons, the glossiness makes defects such as dirt more obvious so that the defect can be seen and repaired before the topcoat layers are applied.
The powder coating composition of the invention contains an acid-functional crosslinker that is a reaction product of 1,3,5-tris(2-hdyroxyethyl)isocyanurate and a cyclic anhydride and a film-forming material reactive with the acid-functional crosslinker. The powder coating may be a clearcoat composition, a basecoat composition, a pigmented, single-layer topcoat composition, or a primer composition.
The powder coating of the invention unexpectedly avoids the contamination and yellowing problems of earlier powder coatings. The reduced yellowing is achieved for composite coatings applied over cathodic electrocoat coatings, wherein the layer obtained with the powder coating of the invention may be a primer layer, a basecoat layer, a clearcoat layer, or a single layer topcoat. The powder coating composition of the invention provides a coating layer with excellent smoothness and gloss, which is especially desirable in a topcoat layer, especially a clearcoat layer. The acid-functional crosslinker of the invention also provides improved pigment dispersion and better color development in pigmented coating compositions.